The purpose of this invention is to provide an improved isotropic broadband measurement of a component of an electromagnetic field. Prior art broadband field measurement system speed and accuracy were limited by existing functional partitioning and circuit architecture. The present invention establishes a faster, more flexible, and more accurate measurement method by improving the overall system functional partitioning, circuit architecture and includes a new calibration approach.
The present invention removes measurement bottlenecks allowing substantially faster field measurement.
The present invention uses data collection and compensation techniques that can be tailored to meet a large variety of measurement conditions and requirements. Measurement accuracy is improved over a very broad dynamic range.
The present invention introduces a highly efficient, accurate and flexible calibration technique. The calibration technique supports a large variety of system data compensation applications and requires very little Probe memory.
A common method of measuring data of an electromagnetic field is based on measuring the electric field component. Referring to FIG. 1, a typical prior art detector-based radio frequency and microwave electric field measurement system 20 contained three primary functional subsystems: (i) a sensing subsystem 22, (ii) a signal conditioning and processing subsystem 24, and (iii) a control and display subsystem 26. These subsystems and the functions performed within them as were commonly partitioned in the prior art are as shown in FIG. 1. Physically, the sensing subsystem 22 and signal conditioning and processing subsystem 24 were often contained within an electric field probe 28 which, in operation is exposed to the environment in which the field is to be measured. The control and display subsystem 26 was contained within a system readout 30 which, in operation is preferably remote from the probe to avoid or reduce perturbation of the field to be measured.
The sensing subsystem 22 included sampling of the incident electric field using a band-limited transducer or antenna for a transducing function 32 followed by a detection function 34 using a diode or thermocouple based circuit. The sensing subsystem 22 included additional filtering components for rf band-shaping and/or noise reduction in a frequency band-shaping function 36. The amplitude of the detection circuit output voltage was coupled into the instrumentation electronics part 24 of the system 20.
Functions performed in the signal conditioning and processing subsystem included: analog signal conditioning such as a filtering function 38 and a level-adjust function 40, conversion of the analog level to digital form in an A/D conversion function 42, and a data communication function 44 to and from the control and display subsystem 26. Measurement compensation in the signal conditioning and processing subsystem 24 included a sensor linearity correction function 46, a temperature compensation function 52 (if required), a signal averaging or data smoothing function 50, and calculation of the electric field amplitude in a composite field calculation function 48. It is important to understand that the data sent from the signal conditioning and processing subsystem 24 to the control and display subsystem 28 typically contained calibrated measured data in common units such as volts per meter (V/m).
The control and display subsystem 26 typically contained a data display function 54 for visual review of instrument state and measured data, a ranging/zero control function 56 for adjusting ranging and zeroing, and sometimes included data logging function 58 along with a user interface function 60 and a data I/O interface function 62 for data exchange between the sensing and display subsystems 22-26, and between the display subsystem 26 and an external data collection system (not shown here). In the operation of prior art system 20, the user, through the system readout 30, requested and read calibrated field intensity data from the electric field probe part, either for a single axis, or for the vector sum of the three axes readings.
Because of the functional partitioning and circuit architectures used, there were some inherent performance limitations associated with such prior art systems as follows:
1. Performing data compensation functions such as linearity and temperature correction in the probe often required additional circuitry for data compensation and added to the micro-controller processing requirements. Adding circuitry increased the physical volume of the sensing subsystem. Furthermore, additional processing slowed down the measurement response time.
2. Calculating the field intensity in the instrumentation electronics of the signal conditioning and processing subsystem also required additional processing time, again slowing down measurement response times. For 3-axis sensors, the field calculation had to be completed for each sensor axis and then the electric field was calculated as a vector sum of the individual field values.
3. Obtaining individual field readings for each of the three measurement axes usually required multiple reading requests from the system. This increased total measurement time and/or introduced inaccuracies when measuring time-varying fields since the individual axis readings were usually not simultaneous.
Common linearity correction techniques included using analog diode voltage compensation and piece-wise linear approximation lookup tables. Analog corrections offered advantages of minimal measurement time impact, but were relatively inaccurate compared to digital techniques. Also, such prior art approaches usually performed well only over relatively narrow dynamic ranges. Beyond those ranges, measurement inaccuracy increased quickly. The additional circuitry required also increased the physical volume required to contain the electronics. Increased circuit volume created a larger cross-section instrumentation electronics enclosure, which perturbed the measured field more and consequently decreased accuracy of measurement at higher frequencies.
The lookup-table method involved characterization of the detector performance at discrete electric field levels during the calibration process. A table of corrected field readings was typically stored in electronic memory. When a measurement was made the detector output voltage was compared to the available correction points and a piece-wise linear interpolation was made to find the compensated electric field reading. Accuracy was limited by the lookup table point resolution, the linear interpolation error, and the numeric precision used to store and calculate the results. The time required to perform the compensation added to the total time required for such a prior art probe to perform a measurement.
The present invention has improvements over the prior art by providing a faster and smaller probe exposed to the field, and provides faster and simpler data acquisition through the process of transmitting uncalibrated data (hereinafter xe2x80x9cRaw Dataxe2x80x9d) from an improved probe (hereinafter xe2x80x9cProbexe2x80x9d) to an improved system readout (hereinafter xe2x80x9cReadoutxe2x80x9d), while enabling the Readout to calibrate the Raw Data by an improved method that calibrates and linearizes the electromagnetic field data with improved accuracy and reduced electronic storage requirements. The improved system has the capability of operating with a variety of Readout configurations and in one or more modes selectable by a user, giving greater flexibility than was available in the prior art.